1. Field of the Invention
The present invention relates to an imaging apparatus, an imaging method, and an imaging control program which generate a synthesized image signal having a wide dynamic range by synthesizing a long-exposure-time signal and a short-exposure-time signal.
2. Description of the Related Art
In a related-art imaging apparatus using a solid-state imaging device such as a CCD (Charge Coupled Device), etc., the amount of light input (amount of exposure) into an imaging device is adjusted by an aperture and en electronic shutter speed. That is to say, the amount of exposure is decreased when a light scene is shot in order to prevent the occurrence of so-called overexposure, which is caused by the saturation of the output signal. On the contrary, the amount of exposure is increased when a dark scene is shot in order to prevent the occurrence of so-called underexposure.
However, when shooting a scene having a great difference in lighting (shooting in backlight, shooting indoors and outdoors simultaneously), there is a problem in that it is difficult to properly reproduce both a light part and a dark part. This is because overexposure occurs at the light part because of saturation, and underexposure occurs at the dark part by only adjusting the amount of exposure on account of a shortage of a dynamic range of a solid-state imaging device to be used.
In order to address this problem, for example, the following method has been disclosed in Japanese Unexamined Patent Application Publication No. 06-141229 (corresponding U.S. Pat. No. 5,455,621). In this method, two different electronic shutter speeds are used in a field, or an electronic shutter speed is changed for each field to capture information of a light area and information of a dark area separately, and the individual pieces of information obtained are synthesized to one image.
Also, as an application of the above, there is an apparatus (a wide-dynamic-range camera) capable of capturing an image having a wide dynamic range. An apparatus and method of improving the quality of the synthesized image has been disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2002-84449 and 2004-120205 (corresponding US publication 2006-033823A1).
In general, as shown in FIG. 13A, an imaging device has a characteristic in which an output voltage is saturated at a certain level with respect to the amount of incident light. As shown in FIG. 13B, the saturation level of individual pixels has variations in an overall screen. Thus, there is a problem in that the video looks uneven on the screen.
If a dynamic range is widened by synthesizing a long-exposure-time signal and a short-exposure-time signal which have variations in the saturation level for each pixel as shown in FIG. 13C, the saturation unevenness of the synthesized signal appears not only at a high-luminance side caused by the short-exposure-time signal, but also in a middle-luminance area caused by the long-exposure-time signal as shown in FIG. 13D.
For a method of reducing the saturation unevenness, the following methods are available. One of the methods is to clip peaks of the output of an imaging device, and then to perform AD (Analog-to-Digital) conversion. Another method is to lower the upper limit value to be subjected to AD conversion than the level at which the saturation unevenness occurs. Still another method is to clip the signal not lower than a certain level of the digital signal after AD conversion.
For example, Japanese Unexamined Patent Application Publication No. 09-200621 has disclosed a solid-state imaging apparatus which is inexpensive, has little saturation unevenness, and has a wide dynamic range using one solid-state imaging device. In the imaging apparatus, a solid-state imaging device is driven at a speed n (n is an integer of 2 or more) times a usual speed, n shutter pulses are supplied in one field to finely control to obtain an image output with continuous gradations, the image output is subjected to white clipping and then stored into n pieces of field memories, the image output is synchronized with a vertical synchronization pule VD and then is added up by an adder to be output.
Also, Japanese Patent No. 3642245 (corresponding U.S. Pat. No. 6,972,800) has disclosed a solid-state imaging apparatus in which an improvement has been made of gradations on the synthesized part of two kinds of video signals as follows. The solid-state imaging apparatus, which widens a dynamic range by synthesizing two kinds of video signals having different amounts of exposure, has a function of setting the gains of a long-exposure-time signal and a short-exposure-time signal to have different values and setting a knee point of the long-exposure-time signal and a knee point of a short-exposure-time signal to have different values. The solid-state imaging apparatus widens the dynamic range by synthesizing the long-exposure-time signal and the short-exposure-time signal, sets the knee point of the long-exposure-time signal lower than a predetermined luminance level, which is a synthesis level, and sets the knee point of the short-exposure-time signal higher than a predetermined luminance level, which is the synthesis level.